View Full Version : Absolute or Differential ?

30th January 2001, 10:42
Hello All,
Can anyone tell me what kind of pressure sensors are normally used in boost gauges? i.e. are they Absolute devices that display a reading relative to zero pressure or are they differential devices that display a reading relative to the surrounding atmospheric pressure?


30th January 2001, 14:03
When the engine is off the reading is 0 bar (0 PSI too http://bbs.22b.com/ubb/wink.gif ) so IMHO it display a reading the relative pressure.

30th January 2001, 15:10
Boost guages are absolute type as they have to show vacuum and pressure . The one in my car is approx 3 bar absolute , ie 2 bar positive pressure .

30th January 2001, 22:39
Well, if it's absolute values than what does negative values mean?
Also what does value of zero means - no air at all?
Sorry mate values are relative to outside pressure of 1 bar - so in a way you can say it's absolute but with shifted values.

Bob Rawle
30th January 2001, 23:43
All these guages record absolute ... its just that their dials are marked as relative values. Watch the zero mark, low pressure needle falls, high pressure needle rises ... so its absolute recorded and relative indicated. BTW only talking about the thickness of the zero mark here.

1st February 2001, 11:40
So they are calibrated to display 0 when they measure 1 bar? Is 1 bar taken as standard atmospheric pressure then?

Andy Tang
1st February 2001, 11:59
1 bar = 1 atmosphere (or there abouts!)

I think that's at sea level!! I can't remember, it's been so long since I've done my A-level in physics!!!


5th February 2001, 22:52
Hi Guys
These gauges use a Bourdon tube (an oval section metal tube formed into a C and sealed at one end). The inside is connected to the manifold whilst atmospheric pressure acts on the outside. As the pressure in the manifold increases under boost conditions, the greater pressure in the tube compared to outside has a tendancy to straighten it.This is recorded as a positive pressure.ie relative to atmospheric pressure acting on the outside of the Bourdon tube. The absolute pressure here would be the gauge pressure plus atmospheric.
When on over-run a partial vacuum is formed in the manifold. This is shown on the gauge as a negative pressure relative to atmospheric.
Hope this clarifies the situation


8th February 2001, 14:14
Hi All!

To the best of my knowledge, all mechanical boost gauges are relative to atmospheric pressure, ie they do not read different on different days with the engine stopped. An absolute gauge will vary slightly with atmospheric pressure but it requires an absolute vacuum reference. I believe some of the HKS electronic gauges use a MAP sensor, they should therefore be absolute.

The markings are generally relative, since anything else would confuse people... imagine you gauge reading 2 bar, you'de be rather scared until you realise it's only actually 1 bar boost :)

Atmospheric varies from day to day and with altitude. Typical values are between 60 and 120kPa (pretty extreme), normally between about 95 and 103kPa. This is another reason why gauges are relative. Imagine being up a mountain and your boost gaue reading vacuum with the engine turned off, it would be rather odd!

Of course, this could all be complete and utter bollox :)